Method for making spark plug shells

ABSTRACT

The invention relates to a method for making high-ductility spark plug shells including the steps of forming a relatively high-density annular billet of about 7.0 g./cc. from a mixture of iron powder and lubricant, sintering the billet at a relatively high temperature of at least 2,400* F. after heating at a substantially lower temperature to drive off lubricant, forming a shell preform from the sintered billet by forward and backward extrusion to produce a density of about 7.6 g./cc., sintering the preform at an elevated temperature in excess of 2,400* F. and extruding the resultant body to form a spark plug shell blank having a density of at least 7.8 g./cc., sintering being performed in a reducing atmosphere.

Hrinevicli, Jr.

Staes ate [54] METHOD FOR MAKING SPARK PLUG FOREIGN PATENTS OR APPLICATIONS SHELLS 752,687 7/1956 7 Great Britain ..75/221 [72] Inventor: John Hrinevich, Jr., Flint, Mich.

Primary ExaminerCarl D. Quarforth [73] Assrgnee: General Motors Corporation, Detroit, Assismm s h f Mlch- Attorney-Sidney Carter and Peter A. Taucher 22 F1 d: 97 1 June 1 0 57] ABSTRACT [21] Appl' 43mm The invention relates to a method for making high-ductility spark plug shells including the steps of forming a relatively [52] US. Cl ..75/214, 29/182, 29/4205, high-density annular billet of about 7.0 gJcc. from a mixture 75/221 75/223 of iron powder and lubricant, sintcring the billet at a relatively 51 int. Cl. .322: 5/00 high temperature of at least F after heating at a [58] Field of Search ..75/221, 223, 214; 29/4205, Manually lower temperature to drive off lubricant, forming a 29/182 2512 shell preform from the sintered billet by forward and backward extrusion to produce a density of about 7.6 g./cc., [56] References Cited sintering the preform at an elevated temperature in excess of 2,400 F. and extruding the resultant body to form a spark UNITED STATESPATENTS plug shell blank having a density of at least 7.8 g./cc., sintering being performed in a reducing atmosphere. 2,315,302 5/1943 Volterra ..75/221 3,331,685 7/1967 Brown et al ..75/22l X 4 Claims, 4 Drawing Figures M l X PRESS BILLET 7.00 g/QC.

PRESINTER LOW TEM PERATU R E SINTER EXTRUDE PREFORM WASH AND RESINTER 'loOg/oc.

EXTRUDE SHELL BLANK 780g/c.c.

PAIENIEBFEB 29 m2 MIX PRESS BILLET I 7.00 g./C.C.

LOW TEMPERATURE PRESINTER sod-900" F.

SI NTER 2400 2475 F 7.10 g tc,

EXTRUDE PREFORM Z60 g/cc.

WASH AND RESINTER 2400+F 7160g/c.c

EXT RUDE SH ELL BLAN K 7.80g/cc.

I NVEN TOR.

B Y Jbhn Hrinevichj:

ATTORNEY METHOD FOR MAKING SPARK PLUG SI'IELLS My invention relates to the formation of spark plug shell blanks by an extrusion process using iron powder. Whilethe use of powder metallurgy is old in the art of forming metal objects of various types and compositions, the formation of such bodies with a high density and high ductility has been a problem where iron powder is used and where the metal is extruded in both a forward and backward direction. It has been found that while iron powder may be extruded without serious difficulty in either a forward or backward direction relative to the movement of the load applicator within the die cavity, the simultaneous forward and backward movement tends to tear the material with the result that the formed body has cracks and defects both on the surface and within the body. One method for avoiding such problem is to use a ferrous metal alloy such as is disclosed in the patent to R. F. Thompson et al., US. Pat. No. 2,882,l90 and in the patent to Reed et al., U.S. Pat. No. 3,067,032. In accordance with my invention, the manufacture of spark plug shells from powdered iron, the shell having characteristics of high density and ductility, is achieved by forming the iron powder into a billet having a relatively high density followed by twice repeated sintering and extruding operations wherein the sintering is carried out at a relatively high temperature as compared with the prior art and wherein forward and backward extrusion is used to produce a body density substantially equal to that of the theoretical density of 7.85 g./cc.

It is therefore an object of my invention to provide a method for forming spark plug shell blanks having high density and high ductility by forward and backward extrusion of iron powder. It is a further object of my invention to provide a method requiring the formation of a starting billet of relatively high density and relatively high sintering temperatures.

These and other objects of my invention may be readily understood from the description which follows as illustrated by the attached drawing in which FIG. I is a flow diagram representing the essential steps of my method;

FIG. 2 shows the annular billet initially pressed from the iron powder;

FIG. 3 shows the extruded shape of the preform resulting from extrusion of the billet shown in FIG. 2; and

FIG. 4 shows the final extruded shape of the spark plug shell blank.

The use of powder metallurgy processes for forming metal articles of various shapes and types is known to be a preferred method of manufacture wherever possible in view of the rapidity of the manufacturing process, its relative simplicity, and the relatively low cost involved. The application of such processes to the manufacture of spark plug shells has not been practical up to this time in view of the relatively complex shape of the shell as shown in the finished shell blank S in FIG. 4. The extrusion of such a shape requires both the forward and backward movement of the metal in the die. Such bidirectional movement of iron tends to tear the material in the die with the result that the formed body must be rejected. I have found that this tearing phenomenon results from the fact that the material does not have the required ductility to lend itself to such rigorous forming operation. While a nickel, copper, or aluminum alloy of iron powder may be used, this is unacceptable in my application in view of the additional cost of materials involved. In the manufacture of spark plug shells in accordance with my invention, I am able to use unalloyed iron powder which is substantially about 99 percent iron with the balance being the normal impurities found in such powder.

As shown in FIG. 1, l have found that processing involving the pressing of a starting billet from the powder, the billet having a density of at least 7.0 g./cc., followed by sintering and extruding steps involving a higher than normal sintering temperature and a backward and forward extrusion, produces the desired state of ductility. More particularly, I have found that the required ductility is achieved when sufficient loading is applied to the iron powder to produce a body having an elongation of more than 27 percent when test bars are pulled on a Rhiele testing machine after sintering at an elevated temperature of at least 2,400 F. in a reducing atmosphere such as dissociated ammonia.

The process of my invention comprises the steps of blending an iron powder with a lubricant commonly used in the art such as zinc stearate or wax and in an amount of about 1 percent by weight. I have found that the iron powder need not be of any particular type such as spherical or spongy but may be of any commercially available type where the material is substantially entirely iron except for normally encountered impurities. The size of the metal powder is less than 300 microns with an average size of at least about 40 microns. When mixed with lubricant, the powder should have little or no tendency to form agglomerates. The blended powder is then loaded into a suitable die cavity in order to form an annular generally cylindrical billet, the loading being such as to produce a billet density of at least 7.0 g./cc. By way of example, a typically dimensioned billet would be about 0.800 inch in diameter and about 0.845 inch in height. The diameter of the centerbore would typically be about 0.330 inch in diameter. The thus formed billet is heated to remove the lubricant, a temperature of from about 800 to 900 F. being found suitable for this purpose over a time period of about 1 hour. The resulting billet is then subjected to a sintering treatment in a reducing atmosphere at a relatively high temperature. I have found that the use of a cracked ammonia atmosphere is suitable, the temperature being at least 2,400 F., preferably from 2,400 to about 2,475 F. While higher temperatures may be used, I do not find the added costs to warrant use of higher temperatures. Sintering for a period of at least about hour has been found suitable to produce a body having a density of about 7.1 g./cc. which may be extruded in both a forward and backward direction without tearing the iron. I have found that the shell preform P shown in FIG. 3 may be formed from the billet B as shown in FIG. 2 by applying a load sufficient to increase the density to 7.6 g./cc. As is common in the art of extrusion, l have found that the extruding operation is assisted by applying a surface lubricant to the billet before insertion in the die cavity. In order to achieve the final shell blank as shown in FIG. 4, I have found it necessary to repeat the sintering and extruding steps. More specifically, the preform is resintered at a temperature of at least 2,400 F. for a period of about %hour. The sintered preform is then inserted into the die cavity having the form of the desired spark plug shell blank and a sufficient loading is applied in order to form the blank with a density of 7.8 g./cc.

In carrying forward the extruding operations and the powder press operations of my invention I have found that it is suitable to use an unheated die and to permit the billet and preform bodies to cool to room temperature prior to extrusion. A loading of from 20 to tons per square inch may be used, it being merely necessary to achieve the body density noted.

As set forth in the foregoing description, I have provided a method for forming a dense, ductile spark plug shell blank starting with unalloyed iron powder. The scope of my invention is as defined in the claims which follow.

What is claimed is:

l. A method for making spark plug shell blanks having high ductility from powdered iron comprising the steps of mixing iron powder with a lubricant, loading said iron powder into a die cavity and imposing a load thereon to form an annular billet having a density of at least 7.0 g./cc, heating said billet to a temperature of from about 800-900 F. for a period of time sufficient to remove said lubricant, sintering said billet for a period of about 45 minutes at a temperature of at least 2,400 F., applying a load to said billet to produce a shell preform having a density of about 7.6 g./cc. by both forward and backward extrusion of said billet in the die cavity, sintering said preform for a period of about 45 minutes at a temperature in excess of 2,400 F and applying a load to said preform to produce a shell blank having a density of at least 7.8 g./cc. by both forward and backward extrusion of said preform in the imposed are from about 20 to tons per square inch and without the addition of heat to the material being pressed.

4. A method in accordance with claim 3 wherein said lubricant is added in an amount of about 1 percent by weight of said iron powder. 

2. A method in accordance with claim 1 wherein said sintering temperature is about 2,425* F. and the reducing atmosphere is cracked ammonia.
 3. A method in accordance with claim 1 wherein the loads imposed are from about 20 to 80 tons per square inch and without the addition of heat to the material being pressed.
 4. A method in accordance with claim 3 wherein said lubricant is added in an amount of about 1 percent by weight of said iron powder. 